That outraged some who have specific ideas about how a prime minister should behave, but others saw it as Mr Key showing he did not take himself too seriously.

Certainly, there have been few prime ministers so willing to discuss their vasectomy, what they do in the shower and the other personal trivia that have been grist to the mill for reporters' colour pieces for several years.

He has said he is leaving primarily for family reasons, as while they have benefited from his position, they have also had to cope with the downsides.

His departure will inevitably raise questions about whether there is another reason he is leaving so abruptly - that was put to Mr Key directly, but he just repeated that he was no "career politician", and that he and his family had had enough.

His family has very much been in the public eye, but even when their actions have brought criticism of him, he has always maintained that they were their own people and were free to make their own choices.

Mr Key, with his wife, Bronagh, and son, Max. He also has a daughter, Stephie. Photo: RNZ / Diego Opatowski

The only time he was publicly disapproving was over comments his son, Max, yelled out at cyclists from a moving car then posted on social media.

There is a double-edged sword, however: if a politician draws his family into the political sphere and they become part of his public image, family members can be more easily drawn into the political debate.

Now attention will turn to who will take over as leader, and as Prime Minister.

It is a very short timeframe, with a special caucus meeting scheduled for 12 December, which should minimise the damage a long drawn-out contest among its MPs could cause.

But it will be a very different National Party heading into next election, whoever its leader, and one the opposition will already be thinking they have a better of chance of beating.